There has been a great deal of interest in nanotechnology over the last decade. Particularly, quasi-one dimensional structures with two dimensions in the nanoscale (e.g. width and height) are fascinating for fundamental research and potential application in commercial products. These structures include carbon nanotubes, such as single-wall nanotubes (SWNT) and double-wall nanotubes (DWNT), and semiconducting nanowires and nanostructures. The SWNT and DWNT are hollow with all carbon shells. SWNT are promising for sensing, transistor, and logic application. DWNT are promising materials for long lasting field emission components for display technologies. Non-carbon nanotubes have also been developed including boron nitride nanotubes. Nanowires, nanoribbons, nanorods, and similar nanostructures include silicon, germanium, gallium nitride, zinc oxide, tin oxide, indium oxide, and other III/V, II/VI, and group IV structures. Most of these structures can be doped and modified to further increase their applicability.
The synthesis of these nanostructures is most often preformed in hot wall reactors, such as quartz or alumina tube furnaces. In this process the reactor walls, substrate, precursor materials, and any catalyst are all heated to high temperatures.
Two examples of the use of hot wall reactors for the synthesis of nanostructures are disclosed by Dai et al. for the synthesis of SWNT from catalyst islands and for the synthesis of MWNT towers, both by hot wall reactor CVD. The SWNT process is disclosed in U.S. Pat. No. 6,346,189, entitled “Carbon Nanotube Structures Made Using Catalyst Islands”, where iron oxide on alumina is deposited on substrates as catalyst islands. SWNT are grown between islands and on the substrate. In U.S. Pat. No. 6,232,706, entitled “Self-Oriented Bundles of Carbon Nanotubes and Methods of Making Same”, Dai describes the synthesis of MWNT on iron thin films on porous silicon. Towers of vertically aligned MWNT are grown.
Cold wall reactors have been used to fabricate multi-walled carbon nanotubes (MWNT) by plasma enhanced CVD (PECVD). The plasma is in direct contact with the substrate and can heat the substrate to the required temperature, as well as ionize the gas precursors for efficient MWNT synthesis. One advantage is vertical alignment of the MWNT with the electric field created by the plasma. While these processes are cold wall, they require complicated plasma equipment and formation and have only been successful in MWNT. It is likely that the conditions are too harsh for other nanostructures, such as SWNT or DWNT.
A different and non-global heating approach is described in two articles: Alexandrescu et al. “Synthesis of Carbon Nanotubes by CO2-Laser-Assisted Chemical Vapor Deposition”, Infrared Physics and Technology, 44, (2003), 43-50; and Rohmund et al. “Carbon Nanotube Films Grown by Laser-Assisted Chemical Vapor Deposition”, Journal of Vacuum Science and Technology B, 20, (2002), 802-811. In this approach described by these articles, a single-CO2 laser is used to locally irradiate (heat) an area on a substrate in the presence of catalytic species and hydrocarbon gas. The laser heats the substrate. The laser also heats the gas through the use of C2H4 included with the gas, which absorbs some of the CO2 laser radiation and heats other, non-absorbant gases within the gas by collisional energy exchange. This technique produces MWNT films of varying yields and defect densities, as well as occasional SWNT. Unfortunately, a drawback of this approach is that the yield and quality are not well controlled. The nanotube synthesis is not uniform, most likely due to difficulty in controlling the laser heating of the substrate, which is the primary heating source. Rohmund uses a secondary heating source of the substrate, to raise the starting temperature of the substrate. The laser is the primary heating source used for nanotube synthesis. While this method does have some of the advantages of a non-hot wall reactor, it requires complicated laser heating of the substrate and catalyst, is difficult to control heating, requires selected substrate materials, and is limited in applicability.
Finnie et al. (Journal of Vacuum Science and Technology A, 22, 2004, 747) describes the synthesis of SWNT by using a direct current to heat the samples themselves, on which catalyst particles are deposited. The substrates were unpatterned silicon with iron oxide nanoparticles. Englander et al. (Applied Physics Letters, 82, 2003, 4797) uses resistive heating of silicon microbridges with metal catalyst thin films to synthesize silicon nanowires and MWNT. The microbridges, which are several microns wide, experience localized Joule heating as their reduced size makes them more resistive than the large silicon connecting pads. Wirebonds are used to connect to the large silicon pads and provide the voltage source. Chiashi et al. (Chemical Physics Letters, 386, 2004, 89) describes a similar method for the synthesis of SWNT with ethanol vapor and Joule heating of a silicon substrate to 850 degrees Centigrade. The catalyst, Fe/Co on zeolite, is deposited on the silicon, and the silicon substrate is clamped on two opposing sides. An ac voltage of typically 6 V is applied to the silicon. While these methods have the advantage of cold wall reactors such as only heating the sample and not the entire reactor, capability of rapid heating and cooling, and possible localization of heating, there are some limitations. The substrate is limited to a conducting or semiconducting material, such as silicon, in order to effectively heat it by Joule heating. Silicon is the only material used, and this will likely restrict the versatility of the technique. The clamps or electrodes used to supply the voltage source will take up a given area on the substrate, and hence reduce the effective space for nanostructure synthesis. There is also a possibility of contamination due to the extreme local nature of the clamps, wirebonds, or electrodes.
In U.S. Pat. No. 6,181,055, entitled “Multilayer Carbon-Based Field Emission Electron Device for High Current Density Applications”, Patterson et al. discloses a carbon deposition process for carbon films via a CVD or physical vapor deposition (PVD) method. This method is solely used for carbon thin film deposition with no attempt to make any well organized arrangement of carbon atoms. Hence, it is used for amorphous carbon films and not relevant for nanostructure synthesis.
In U.S. Pat. No. 6,692,568, entitled “Method and Apparatus for Producing MIIIN Columns and MIIIN Materials Grown Thereon”, Cuomo et al. describes a sputter transport method used to produce arrays or layers of columnar structures of single crystal Group III nitride on substrates. The substrate rests on a substrate holder that is also the primary anode for sputtering. The substrate holder can be heated or cooled in order to modify the temperature of the substrate. The material for column growth is provided by sputtering techniques which has limitations if one were to attempt to extend the process for nanostructure synthesis. The need for sputtering of a solid target is complicated, and the sputtering sources are limited to certain materials.
Another method of cold wall CVD is through halogen lamp rapid thermal heating. In this process, the halogen lamps are heated and irradiate the sample or sample holder with infrared radiation (IR). The lamps are usually, but not always, outside of the reactor. This is a cold wall technique. However, it is limited in use as it requires radiation heating which limits its temperature maximum, often involves halogen lamp burnout, and requires either the substrate or substrate holder to absorb the IR. There are also design limitations on the chamber and overall process.
The synthesis of MWNT in cold wall chambers is described in two articles: Wei et al. “Directed Assembly of Carbon Nanotube Electronic Circuits by Selective Area Chemical Vapor Deposition on Prepatterned Catalyst Electrode Structures,” Journal of Vacuum Science and Technology B, 18, (2000), 3586-3589; and Avigal and Kalish “Growth of Aligned Carbon Nanotubes by Biasing During Growth.” Wei et al. describes the synthesis of MWNT with iron thin films patterned on oxide coated silicon wafers. The substrate is heated to 660 degrees Centigrade in a stainless steel chamber and acetylene is used as the carbon feedstock. Avigal and Kalish describe a quartz cold wall reactor with a molybdenum substrate holder used to heat the substrate and an electrical bias on the substrate. The bias is required for MWNT growth. While these techniques are used to synthesize MWNT with diameters as low as 10 nm, it is unclear whether they will work for other nanostructures. There are limitations in nanostructure growth, quality, and defect density.
Lan et al. (Advanced Functional Materials, 14, 2004, 233) presents the synthesis of gallium nitride and indium nitride nanorods grown on gallium nitride nanowires. The growth of gallium nitride is done with conventional quartz tube furnace methods. Lan also describes the MOCVD synthesis of the indium nitride nanorods with a cold wall quartz tube chamber and a substrate heater, where the growth occurred at 500 degrees Centigrade. The MOCVD cold wall system described in the article will likely be limited in application. A quartz chamber is only suited for relatively low temperatures, that is much lower than most nanostructure synthesis processes. In addition to flexibility issues with processing, there are also safety concerns such as radiation through the quartz when the sample is at sufficiently elevated temperatures. There are also maximum temperatures that the quartz chamber can withstand without the addition of significant heat shielding.